1. Field of the Invention
The present invention relates to an optical-diffusion correcting device for a television camera tube and more particularly to a device for correcting diffusion effects of the periphery.
2. Description of the Prior Art
Since television camera lenses cover a field which is larger than the field of the useful image or image to be televised, it is known that defects appear as a result of diffusion of light rays in that portion of the image which is supplied by the camera lens and which surrounds the useful image. This phenomenon of diffusion of the periphery in the useful image is also known as peripheral diffusion. In point of fact, the correcting devices which continuously measure the mean content of the useful image and apply a fraction of this value as a correction in the video signal transmission channel are not capable of detecting and therefore correcting peripheral diffusion defects in the useful image.
It has been sought to remedy the above-mentioned diffusion defects at the periphery of the useful image. Thus a known method consists in placing a mask provided with a window in front of the scanning tube target. The unwanted portion of the image supplied by the camera lens to the scanning tube target can therefore be theoretically eliminated. In practice, however, the design of the scanning tube makes it impossible to place the mask immediately against the target since a glass pastille a few millimeters in thickness is placed in front of the target on the tube faceplate in order to prevent the halos which would be produced by any parasitic reflections in the faceplate. As a result of said pastille, the distance between the mask and the focal plane of the camera lens or in other words the target plane makes it necessary to enlarge the window of the mask in order to ensure that no shadow is produced on the edges of the useful image. The mask is therefore of limited effectiveness since the target receives part of the unwanted image.